The availability of heating and cooling is fundamental to survival and comfort. Thermal transfer devices, including heat pumps and air conditioners, introduce power from an external source to supply or remove heat as desired, and nearly invariably employ a transfer medium to effect this exchange. The transfer medium (also called the working medium or fluid, and often referred to as a refrigerant) that has been found historically to be most cost effective during the ordinary vapor compression refrigeration cycle is that of a group of halogenated hydrocarbons containing one or more fluorine atoms, available under the trademark FREON. In recent years at least such compositions that are chlorinated have been linked to the destruction of the Earth's protective ozone layer, and have been identified as one of humankind's most serious and urgent environmental problems. Consequently, countries throughout the world have mandated that the use of such compositions be significantly reduced and, by the beginning of the next century, eliminated.
Existing heat transfer devices are subject to a variety of other shortcomings. Commonly such devices are closed systems that employ reciprocating or displacement type engines, which have relatively low efficiencies and a large number of parts. For example, vapor compression refrigeration cycle-based systems require one or more refrigeration coils, compressors, condensers and expansion valves or other throttling equipment. The number, configuration and complexity of parts and their relative motions result in devices that are expensive to manufacture, are subject to significant wear and require appreciable maintenance. Their size and weight make them undesirable for applications where compactness, low weight and higher efficiency are more critical, such as on aircraft and other vehicles.